In association with the continuous cooking of chemical cellulose pulp in a vapour phase digester, pre-treated chips are fed to the top of the vapour phase digester as a mixture of chips and liquid. The mixture of chips and liquid undergoes a dewatering process at the top of the digester in what is known as an “inverted top separator”, which feeds the mixture in an upwards direction. An example of such an inverted top separator is shown in SE 511850. Chips are fed vertically upwards in the inverted separator with the feed screw. The major part of the mixture of chips and liquid is withdrawn through vertically arranged withdrawal strainers.
When the chips pass the upper edge of the separator, they fall down through a vapour phase at the top of the digester. The chips subsequently become located uppermost in a column of chips that is not submerged in liquid. However, a liquid level is established under the column of chips. Steam is added directly to the vapour phase.
A number of withdrawal strainers are arranged under the liquid level for the withdrawal of liquor. These are arranged primarily for the purpose of top, intermediate and bottom circulation flows. The upper strainer (not mentioned in the patent) is arranged such that it can increase locally the liquid/wood ratio at the top of the digester, such that it is possible in this way to adjust the alkali concentration in the digester. This upper strainer is usually also denoted as the “trim screen”.
The withdrawal of liquid from the fluid level through withdrawal strainers that is subsequently sent to the preceding chips treatment stage in the digester system is known. SE 504 644 C2 shows how liquor is withdrawn at different levels in the digester through upper, intermediate and lower withdrawal strainers 26, 27, 28.
The upper withdrawal strainer 26 is not described in detail in the patent, but this is an adjuster strainer or a trim screen, where the purpose of the withdrawal is primarily to equilibrate the concentration of alkali in the cooking process, and to adjust the temperature, by increasing the liquid/wood ratio at the top of the digester.
It is not unusual in the continuous digesters used today that a digester that is dimensioned for around 500 tonnes/day is run at a higher load, giving a production capacity of as much as 1,500 tonnes/day. This leads to the top separators being underdimensioned, and unable to cope with the withdrawal of the large quantities of liquid that are required in order for it to be possible to operate the cooking process in an advantageous manner. This leads to it being necessary to exchange the top of the digester for a top with a larger diameter and a larger top separator, in order to cope with the new conditions. Not only is this a very expensive measure, it is also work that take a long time and gives rise to extended interruptions in operation.
A first aim of the invention is to offer a method and a vapour phase digester that partially or fully solves the problems and disadvantages of today's continuous digester plants, where digesters are run at excess load, as described above in the summary of the prior art technology.
A second aim is to achieve a method and a vapour phase digester in order to reduce the liquid/wood ratio at the top of the vapour phase digester.
A third aim is to offer a solution for dewatering the top of the digester in a vapour phase digester, without needing to exchange existing top separators.
A fourth aim is to offer a vapour phase digester that can be run at more advantageous process conditions.
The objects described above are achieved with the method and with the vapor phase digester according to the present invention.
The suggested invention offers a method and a vapour phase digester in which the principal aim of the invention is to reduce the liquid/wood ratio at the top of a vapour phase digester in a continuous digester plant.
This is achieved through the arrangement of at least one withdrawal strainer at the liquid volume of the digester close to the liquid surface, where the chips have had only a short retention time in the liquid volume. Liquid is withdrawn from the withdrawal strainer and sent to preceding chips treatment stages.
The withdrawal strainer acts in principle as an extra top separator that allows a dewatering at the top of the digester. The upper edge of the withdrawal strainer is arranged at a distance under the liquid surface, where the distance is less than 0.5× the diameter of the withdrawal strainer. This can be expressed in an alternative manner: the withdrawal takes place at a distance where the chips have had a retention time in the liquid part of 0.1-15 minutes.
The following positive properties relative to prior art technology are achieved with the invention:                the liquid/wood ratio at the top of the digester is considerably reduced.        the vapour phase digester can be run at a higher loading.        under dimensioned vapour phase digesters can be rebuilt such that they comprise a withdrawal strainer as specified by the patent claims, to a low cost relative to what the cost would be for exchanging the complete top separator.        